1. Field of the Invention
This application relates generally to optical coherence tomography (OCT), and more specifically, to angiographic OCT.
2. Description of Related Art
Angiographic optical coherence tomography (OCT) is a technique that visualizes vasculature based on three dimensional (3D) OCT volume information. The underlying concept of OCT angiography is that the blood flow inside vasculature induces motion that manifests as changes in relative phase and/or pixel intensity in OCT imaging over time. OCT angiography processes visualize such phase and/or intensity changes to represent vasculature. Such changes are assumed to originate from blood flow and are, therefore, assumed to represent vasculature. 3D OCT can be used for angiographic purposes, as it possesses sufficient resolution for visualization of capillary structures and it can provide depth-resolved information. Flow, and vasculature by extension, can be visualized not only as an en-face projection, but also depth by depth or, more generally, cross-section by cross-section. Furthermore, different ranges of depth can be conveniently integrated to provide en-face angiographic visualizations corresponding to different zones along depth (e.g., superficial blood vessels, deep capillary plexus, choriocapillaris), all from a single 3D OCT scan process (including those involving repetitive scans, for example, as described below) with a scan time on the order of only several seconds. Compared with conventional angiography imaging modalities (e.g., fluorescein angiography (FA), and indocyanine green angiography (ICGA)), OCT angiography does not require exogenous dye which can induce adverse reactions in patients. In addition, the high signal to noise ratio (SNR) of OCT technology and high sensitivity and high contrast for flow detection enable OCT angiography to provide noninvasive, and high resolution/fidelity visualization of vasculature in both transverse and depth directions.
Generally, angiographic OCT techniques may be implemented by: (1) repetitively scanning each location within a 3D volume and analyzing the multiple repetitions at each scan location for motion; (2) incremental stepping between locations (i.e., not repetitively scanning each location), such that each location is sufficiently similar to the previous location to enable motion detection analysis; and (3) scanning at a very high resolution in the fast axis, such that there is overlap between successive A-lines; therefore, rather than comparing between corresponding locations in B-scans, adjacent A-lines can be compared to evaluate motion. Variants of the above methods have also been proposed, such as measuring several A-lines and then repeating before moving onward within the B-scan.
One example approach is optical microangiography (OMAG), which utilizes both OCT phase and magnitude information to deliver finely detailed angiographic images. Recently a variation of OMAG called Intensity Differentiation, which does not utilize phase information, has been proposed. Another example is split-spectrum amplitude-decorrelation angiography (SSADA), which uses only magnitude information in which spectral data is split into chunks that are separately processed based on an amplitude-decorrelation formula, and then later combined.
In OMAG, calculations are based on differences between intensity values. These difference calculations are implemented as subtractions between intensity terms, and in the case of complex operations, may be followed with taking the magnitude of the result.
With SSADA, the spectral bandwidth is split into smaller equally-sized bands. This is illustrated in FIG. 1, whereby the total bandwidth (BW) is split into four sub-bands (bw1, bw2, bw3, and bw4) to produce four interferograms I′(x,k′) corresponding to each of the four sub-bands. For each sub-band: (1) A window function is applied; (2) Images are constructed; and (3) Angiographic calculations, using an amplitude-decorrelation formula, are performed between adjacent frames. Then, the decorrelations among all frame combinations are averaged. B-scans that might not match the other B-scans may be excluded. This can serve to reduce motion artifacts that manifest as bright lines that may span an en-face angiogram from one end to the other.